Backflow in a water distribution system can cause contamination of the potable water supply. Backflow can be caused by back siphonage when the pressure in the potable supply pipe or main drops, creating a vacuum, which causes a backflow of possibly contaminated liquid from the service pipe. Backflow can also result from back pressure, when the pressure in the service pipe exceeds that in the potable supply pipe.
Control of both types of backflow requires installation of an air gap when feasible or a backflow-prevention assembly. An air gap eliminates backflow, but it can be bypassed. Backflow-prevention assemblies of several types are also used. Two common types of assemblies are the reduced-pressure assemblies (RPBA's) and the double check valve assemblies (DCVA's). Commonly used to prevent health hazards, RPBA's have two independent check valves, with a pressure differential relief valve located between them. Contamination of the potable water supply is prevented by draining the chamber between the check valves. Double check valve assemblies (DCVA), commonly used with non-health hazards, have two check valves between two shutoff valves. A RPBA or DCVA is installed at the inlet to the service connection.
The presently available backflow prevention devices stop the reverse flow of water during a situation involving negative pressure conditions in the main water supply. However, the presently available devices are not effective in relieving high pressure conditions. For instance, when a water heater relief valve fails, the expansion of overheated water causes backflow of water in the water service line. This will cause the RPBA or DCVA to close; then, if the pressure continues to rise, the water heater may explode.